e i c n introgression of crop alleles into wild or weedy

ES44CH17-Ellstrand ARI 25 September 2013 10:32

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Introgression of Crop Allelesinto Wild or WeedyPopulationsNorman C. Ellstrand,1 Patrick Meirmans,2 Jun Rong,3

Detlef Bartsch,4 Atiyo Ghosh,5 Tom J. de Jong,6

Patsy Haccou,7 Bao-Rong Lu,8 Allison A. Snow,9

C. Neal Stewart Jr,10 Jared L. Strasburg,11

Peter H. van Tienderen,12 Klaas Vrieling,13

and Danny Hooftman14

1Department of Botany and Plant Sciences, University of California, Riverside,California 92521; email: [email protected] voor Biodiversiteit en Ecosysteem Dynamica, Universiteit van Amsterdam,1098 XH Amsterdam, The Netherlands; email: [email protected] for Watershed Ecology, Institute of Life Science and Key Laboratory of Poyang LakeEnvironment and Resource Utilization, Ministry of Education, Nanchang University,330031 Honggutan Nanchang, People’s Republic of China; email: [email protected] Office of Consumer Protection and Food Safety, 10117 Berlin, Germany;email: [email protected] Systems Biology, Okinawa Institute of Science and Technology,Okinawa 904-0495, Japan; email: [email protected] of Biology Leiden, Leiden University, 2333 BE Leiden, The Netherlands;email: [email protected] University College The Hague, Leiden University, 2514 EG The Hague,The Netherlands; email: [email protected] of Education Key Laboratory for Biodiversity and Ecological Engineering,Department of Ecology and Evolutionary Biology, Fudan University, Shanghai 200433,People’s Republic of China; email: [email protected], [email protected] of Evolution, Ecology, and Organismal Biology, The Ohio State University,Columbus, Ohio 43210; email: [email protected] of Plant Sciences, University of Tennessee at Knoxville, Knoxville,Tennessee 37996; email: [email protected] of Biology, University of Minnesota-Duluth, Minnesota 55812;email: [email protected] voor Biodiversiteit en Ecosysteem Dynamica, Universiteit van Amsterdam,1090 GE Amsterdam, The Netherlands; email: [email protected] of Biology, Leiden University 2333 BE Leiden, The Netherlands;email: [email protected] for Ecology and Hydrology, National Environmental Research Council, Wallingford,Oxfordshire OX10 8BB, United Kingdom; email: [email protected]

17.1

Review in Advance first posted online on September 9, 2013. (Changes may still occur before final publication online and in print.)

Changes may still occur before final publication online and in print

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Annu. Rev. Ecol. Evol. Syst. 2013. 44:17.1–17.21

The Annual Review of Ecology, Evolution, andSystematics is online at ecolsys.annualreviews.org

This article’s doi:10.1146/annurev-ecolsys-110512-135840

Copyright c© 2013 by Annual Reviews.All rights reserved

Keywords

gene flow, hybridization, plant conservation genetics, transgene flow,evolution, invasiveness

Abstract

The evolutionary significance of introgression has been discussed fordecades. Questions about potential impacts of transgene flow into wild andweedy populations brought renewed attention to the introgression of cropalleles into those populations. In the past two decades, the field has advancedwith considerable descriptive, experimental, and theoretical activity on thedynamics of crop gene introgression and its consequences. As illustrated byfive case studies employing an array of different approaches, introgressionof crop alleles has occurred for a wide array of species, sometimes with-out significant consequence, but on occasion leading to the evolution ofincreased weediness. A new theoretical context has emerged for analyzingempirical data, identifying factors that influence introgression, and predict-ing introgression’s progress. With emerging molecular techniques and anal-yses, research on crop allele introgression into wild and weedy populationsis positioned to make contributions to both transgene risk assessment andreticulate evolution.

HISTORICAL PERSPECTIVE

Most crops were domesticated from wild plants centuries or even millennia ago (Warwick &Stewart 2005). Early evolution under anthropogenic selection produced domesticated plants thatwere locally adapted and more productive. With the advent of formal plant breeding, progress inplant improvement depended on selecting from (a) pre-existing variation in the evolving crop and(b) alleles obtained by intentional hybridization with the crop progenitor and other wild or weedy(henceforth, WW) relatives. Recently, techniques available to breeders have expanded to includemethods such as human-mediated intertaxon crosses, hybrid embryo rescue, protoplast fusion,induced mutations, and transgenesis. Still, spontaneous and intentional hybridization betweenWW populations and locally adapted landraces continues to be a source of variation for cropimprovement both in formal breeding and in traditionally managed agroecosystems (Hajjar &Hodgkin 2007, Jarvis & Hodgkin 1999).

Conversely, spontaneous hybridization can be the first step for the flow of novel crop allelesin the other direction, i.e., into WW relatives (Ellstrand 2003). Subsequent establishment ofthose alleles is known as introgression, “the permanent incorporation of genes from one set ofdifferentiated populations (species, subspecies, race, and so on) into another” (Stewart et al. 2003,p. 806). The process of introgression begins when a fertile or semifertile hybrid (or even an F2

or later segregant) successfully backcrosses with one of the parental species. Unless opposed byselection or drift, further introgression proceeds under repeated backcrossing or selfing.

The importance of crop allele introgression in the evolution of WW populations has beencontroversial. In the late twentieth century, deWet & Harlan (1975) recognized peripatric andsympatric wild-weed-crop complexes as zones of significant gene exchange across reproductiveisolating barriers of varying permeability. They argued that two of the three avenues of exchangewould be significant: (a) Such evolutionary hotbeds would favor the flow of beneficial alleles fromthe wild to the crop, providing farmers with a constant source of genetic variation useful for plantimprovement and (b) gene flow from crops into weedy populations could stimulate the evolutionof crop mimics. But their view was that “selection almost completely prevents gene flow in the

17.2 Ellstrand et al.


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direction of the wild race” (deWet & Harlan 1975, p. 106). Others had a contrasting view, e.g.,“. . . gene flow, if it exists, is apparently more effective in the direction from the cultivated to thewild populations” (Ladizinsky 1985, p. 191). At a time when gene flow and hybridization wereoften inferred from morphology alone, rather than genetically based markers, conclusive evidencefor intertaxon gene flow was often lacking.

The flow of crop alleles into WW populations might have remained solely a topic for academicdiscussion; but applied evolutionists subsequently recognize that intertaxon gene flow can leadto evolutionary changes that impact human affairs. Hybridization can sometimes stimulate theevolution of new weeds or invasives (Schierenbeck & Ellstrand 2009) or contribute to the riskof extinction (Ellstrand & Elam 1993, Levin et al. 1996). Given these potential problems, theadvent of genetically engineered crops and subsequent questions about possible consequences oftransgene flow brought renewed attention to the flow of crop alleles into WW populations.

Stimulated by questions regarding transgene dispersal, dozens of evolutionary geneticists andecologists took to the field in the 1990s to conduct experimental or descriptive studies addressingthe likelihood of such gene flow, usually using nontransgenic plants as model systems. Their initialfocus was to determine whether spontaneous hybridization between crops and WW relativesoccurred under field conditions. Secondary questions included whether gene flow occurred atdistances and rates large enough to permit crop genes (and by extension, transgenes) to enter WWpopulations and, if so, what consequences were expected to ensue. Subsequent introgression wasoften assumed, yet introgression-related data, such as the fitness of advanced hybrid generationsand the effects of specific crop traits, were largely neglected. The most frequent relevant data fromthat early research were measurements of the relative fitness of the F1s versus that of the WWparents (Ellstrand 2003).

Ellstrand (2003) reviewed these and earlier relevant hybridization studies. He examined datafor the world’s 25 most important domesticated plants in terms of area planted; for 22 of theseplants, he found substantial empirical evidence for some spontaneous hybridization with WWrelatives somewhere in the world. Hybridization patterns were idiosyncratic for the crops. Forsome, such as coffee, hybridization apparently occurs rarely and in a few locations. For others,low levels of hybridization typically occur over much of the globe; in the case of wheat and itsweedy Aegilops relatives, hybridization occurs whenever the species co-occur in temperate regions.For a few, such as cultivated sunflower, hybridization can occur at relatively high rates. Severalyears later, a book by Andersson & de Vicente (2010) re-evaluated and updated what is knownregarding opportunities for crop gene flow to WW relatives for a similar list of 20 importantfood crops. With rich detail describing the crops’ reproductive and dispersal biology, compatiblerelatives, and hybrid fitness, each case study is presented as a crop-specific guide for transgene flowassessment. Both books (Andersson & de Vicente 2010, Ellstrand 2003) mention introgressionwhen they found supporting data.

Both books focus strongly on hybridization, and hybridization is not introgression. If significantevolutionary or ecological impacts occur, they typically occur through introgression (Arnold 2006).As the significance of introgression became clear, the research focus of the twenty-first centurychanged considerably, and the research emphasis shifted from “Does hybridization occur?” to thenext step of “How and when does introgression occur and at what levels?” The context broadenedfrom focusing on the transgene escape to the introgression of any domesticated alleles (den Nijset al. 2004). The question of “When and how will introgression have any significant impact?”has also become paramount. Although the introgression of domesticated plant genes has receivedattention from publications in the context of transgene flow (den Nijs et al. 2004, Kwit et al. 2011,Stewart et al. 2003), the wider burst of research activity that started this century awaits a thoroughreview.

www.annualreviews.org • Crop-to-Wild/Weed Introgression 17.3


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Thus, we address introgression from domesticated plants to their WW relatives. As mentionedabove, such gene flow provides examples of contemporary microevolution that may have impor-tant implications. Below we ask, “What do we know about crop gene introgression into WWpopulations?” including when evolution by introgression creates a negative impact (Natl. Res.Counc. 2002, Organ. Econ. Co-op. Dev. 1993).

Our specific focus is the establishment of alleles from domesticated plants in WW populations.How alleles are naturally introgressed may follow a variety of pathways. The pollen parent in theinitial hybridization event (or events) could involve a plant in cultivation, a volunteer left froma previous planting or from seed spillage into a natural population, or a recent escape fromcultivation. An alternate evolutionary pathway could start with an uncultivated plant as pollenparent hybridizing with a plant in cultivation. If the hybrid seed from the maternal plant naturallydisperses or is harvested and sown in the same location, it can variously self-pollinate, cross withother hybrids, or spontaneously backcross with its parental parent (depending, in part, on thebreeding system of each of the parental taxa).

We define wild plants as those capable of growing and reproducing in ecosystems thatare largely undisturbed by humans. Weeds are those whose populations persist only underanthropogenic disturbance. Of course, intermediate cases exist; also, some taxa may include bothwild and weedy populations.

First, we present a brief overview of hybridization and introgression in plants. Next, we examinevarious molecular approaches for identifying and studying crop allele introgression. Some casestudies of introgression of domesticated alleles into WW populations follow, including the fewdescribed examples of spontaneous transgene introgression. We conclude with a look to the future.

HYBRIDIZATION AND INTROGRESSION IN PLANTS

Hybridization is an important component of plant evolution, occurring in roughly 25% of plantspecies (Baack & Rieseberg 2007). Nonetheless, hybridization occurs unevenly among plant taxa;a higher propensity occurs in certain genera, families, and orders (Whitney et al. 2010). In afew families, natural intergeneric hybridization occasionally occurs (Stace 2010). Even for readilyhybridizing species, hybridization and introgression rarely occur at high enough rates to jeopardizethe integrity of a species (Levin et al. 1996).

Thus, although hybrids are produced, most of them have no further evolutionary impact. How-ever, sometimes a little hybridization has considerable evolutionarily significance (Arnold 2006).Mechanisms such as alloploidy or apomixis can fix hybridity, leading to the evolution of new hybrid-derived species from one or just a few founders (Arnold 2006). Likewise, homoploid hybrid speci-ation can yield new species without a change in ploidy level or mating system (Buerkle et al. 2000).More frequently, the evolutionary impact of hybridization is mediated through introgression.

Edgar Anderson’s 1949 book Introgressive Hybridization championed introgression as a poten-tially important process for introducing adaptive variation into a population. At that early date,hybrids and hybrid derivatives were largely assigned by morphology. Nowadays, multilocus molec-ular markers have greatly enhanced the ability to detect introgression (Rieseberg & Wendel 1993).The last major review, twenty years ago, examined hundreds of putative cases of introgression,identifying about three dozen cases (almost all bolstered by molecular data) of probable natural in-trogression in plants (Rieseberg & Wendel 1993). Today, that number might be larger by an orderof magnitude (examples below and in Table 1). By definition, introgression cannot be as commonas hybridization. Determining how common introgression in plants is and its real evolutionarysignificance remains challenging and generally requires molecular marker-based methods.



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Table 1 Examples of introgressed crop alleles in wild or weedy populations with strong support

Cultivated Ancestor

Example of wild or weedy (WW) taxon withone or more populations with introgressed

crop alleles ReferenceBeta vulgaris vulgaris, beet Europe’s weed beet (descendant of beet ×

B. v. maritima)Case study in this review

Brassica napus, oilseed rape B. rapa Andersson & de Vicente 2010Cichorium intybus, chicory WW C. intybus Kiaer et al. 2007Cynara cardunculus var. scolymus, artichoke Some populations of California’s artichoke thistle

(descendant of artichoke × C. c. cardunculus)Leak-Garcia et al. 2013

Glycine max, soybean G. soja Andersson & de Vicente 2010Gossypium hirsutum, cotton G. barbadense Andersson & de Vicente 2010Helianthus annuus, sunflower H. petiolaris Case study in this reviewLactuca sativa, lettuce L. serriola Case study in this reviewOryza glaberrima, African domesticated rice O. barthii Andersson & de Vicente 2010Oryza sativa, Asian domesticated rice O. rufipogon Case study in this reviewPhaseolus vulgaris, common bean P. vulgaris Andersson & de Vicente 2010Raphanus sativus, radish R. raphanistrum Case study in this reviewSolanum tuberosum, potato S. edinense (a stabilized clonal hybrid of potato ×

S. demissum)Ellstrand 2003

Sorghum bicolor, sorghum S. halepense Andersson & de Vicente 2010Triticum turgidum wheat Aegilops peregrina Kwit et al. 2011Ulmus pumila, Siberian elm U. minor Ellstrand 2003Zea mays mays, maize Z. m. mexicana Andersson & de Vicente 2010

IDENTIFYING HYBRIDS AND INTROGRESSANTS

Determining whether an individual is a hybrid or has hybrid ancestry is not straightforward in theabsence of genetic markers. Phenotypic intermediacy might indicate an early-generation hybrid,a segregant, or a later generation backcross retaining some characters from both parental species.Also, phenotypic response to environmental variation or developmental instability may createunexpected morphologies that mimic hybrid ancestry. Furthermore, experimental research hasrevealed that, more often than not, for any given morphological or otherwise quantitative trait,F1s are not necessarily intermediate compared to their parents (Rieseberg & Ellstrand 1993).

For detecting introgression, the problem becomes worse. In his seminal book, EdgarAnderson, believing introgression became increasingly important as the number of introgressedalleles decreased, bemoaned:

How important is introgressive hybridization? I do not know. One point seems fairly certain; itsimportance is paradoxical. The more imperceptible introgression becomes, the greater is its biologicalsignificance. It may be of the greatest fundamental importance when by our crude methods we can dono more than demonstrate its existence (Anderson 1949).

Co-dominant genetic-based markers can provide more certainty than phenotypic measure-ments. For hybridizing taxa fixed for the alternate alleles, a1 and a2, if an individual is heterozy-gous for both alleles, the individual can be assigned hybrid ancestry. But information from a singlelocus is insufficient to distinguish an introgressant from a first generation hybrid.



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When few individuals are surveyed, it is often uncertain whether both parental taxa are actuallyfully fixed for alternate alleles. If not, an allele might be wrongly assigned as introgressed. Toillustrate, imagine both putative parents are not fixed for alternative alleles but have retainedalleles from a common ancestor (symplesiomorphy or incomplete lineage sorting). One specieshas the a1 allele at 97% frequency and the a2 allele at 3%; the other has a1 at 1% and a2 at 99%. Ifan individual is found that is heterozygous at that locus, does it have hybrid ancestry or is it just oneof the rare heterozygotes within either species? Thus, thorough sampling of putative parental taxais crucial, especially when few loci are assayed. Populations that have never been in reproductivecontact with crops are particularly useful but are difficult to identify for widespread ancient cropswhose distributions have changed. The accuracy of assigning hybrid ancestry and introgressionincreases for each additional independent marker assayed and as more data are available fromother sources of information.

Alternatively, transgenes, the result of genetic engineering, are evolutionarily unique to crops.If we find an engineered gene in a WW plant, we can be certain that it has hybrid ancestry. Atthe moment, evidence for transgene introgression is known from only a few cases (Warwick et al.2008, Wegier et al. 2011).

In most cases, the ideal tool kit is a set of genetically based markers for large numbers ofloci dispersed throughout the whole genome that can be easily and unambiguously scored. Themost useful markers are codominant, allowing for homozygous loci to be distinguished fromheterozygous loci. Furthermore, access to a set of anonymous loci with no a priori expectations ofselection history can provide a baseline expectation of patterns of introgression to compare with alocus of interest. However, fully dominant molecular markers and/or markers whose genetic basisis unclear have limited value in elucidating ancestry (Whitkus et al. 1994).

Allozymes were the first widely used biochemical marker system, an improvement over mor-phological markers, but they were constrained by the low number of loci sampled. For a giventaxon, polymorphic allozyme loci rarely number more than a dozen, each with two or a few morealleles. DNA-based markers are much more powerful. For example, polymorphic microsatelliteloci may have several to dozens of alleles per locus. Their primary limitation is their relativelyhigh back-mutation rate, resulting in alleles with different histories appearing to be identical(homoplasy). Microsatellite mutation rate should be less problematic for crop allele introgressionresearch when such introgression has occurred recently in evolutionary time, in many cases, lessthan a century (Ellstrand et al. 2010).

Single nucleotide polymorphisms (SNPs) are emerging as highly informative markers for char-acterizing introgression. SNPs can be confidently scored and provide high throughput analysis;SNP microarrays can survey polymorphism at up to tens of thousands of sites across a plant’sgenome and are now available at relatively affordable prices. Alternative methods using reducedgenomic libraries include RADtag sequencing, which might also be useful for larger genomes(Baird et al. 2008). Also, the genotyping-by-sequencing approach allows direct evaluation of large,complex (e.g., polyploid) genomes without prior development of other molecular tools (Elshireet al. 2011). Although such an approach is feasible for any species, currently genome assembly re-mains labor-intensive and costly for many nonmodel species, especially those with large genomes.

Direct SNP genotyping as well as single molecule sequencing are advancing rapidly, andcheaper flexible genotyping methods are becoming available (Maughan et al. 2011). Even se-quencing of polyploid plants is within reach because of single molecule sequencing (Kovalic et al.2012), yielding allele dosage and haplotype information.

With access to a large number of loci of known genome location, variation in intro-gression rates throughout the genome can be estimated. Genome-level approaches have al-ready revealed that introgression occurs unevenly across the genome of the species involved



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(Baack & Rieseberg 2007, Hooftman et al. 2011, Twyford & Ennos 2012). Information on linkagedisequilibrium (LD) between markers can aid in making inferences about introgression because,in cases of recent introgression, tightly-linked taxon-specific markers would still be in LD. LDshould decrease in later generation hybrids, and is expected to be very low if similarity betweentaxa resulted from a common but ancient ancestry—unless selection acting on allele combinationsis extremely strong (e.g., Linder et al. 1998). Linkage information can be obtained by crossing thetaxa under study and analyzing segregation in the F2 or BC1 generation. Because most importantcrops have been already extensively genetically mapped, new markers can easily be placed on thatmap and tested for LD. In particular, haplotype data are ideal for reconstruction of introgressionas they provide information about linkage relationships, that is, whether genes are in coupling orrepulsion phase, which in turn gives information on the ancestral genotypes.

The most detailed genetic information regarding loci and linkage is obtained via genomesequencing. One example of detection of introgression through sequencing is the discovery ofNeanderthal DNA in modern humans (Green et al. 2010) revealed by next-generation sequencingtechniques combined with what is known regarding the migration patterns of modern humans.

MEASURING INTROGRESSION

Once introgression has been identified, more questions follow: What fraction of the recipientpopulation is introgressed? Are introgression rates roughly equivalent across the genome or aresome loci or genomic regions significantly more or less introgressed relative to neutral expecta-tions; that is, does selection appear to be involved in which loci have passed from one taxon tothe next? As the number of marker loci increases, so too will the ability to answer these questions,especially if the genomic location of those loci is known.

Another important factor is the timescale over which introgression has occurred. How manyyears or generations of introgression have occurred? For example, for transgenes the timescale ofintrogression into unmanaged populations is necessarily recent, no longer than since the first fieldrelease of the transgenic crop, obtainable from public regulatory records. However, the detec-tion methodology depends on publically available transgenic sequences. Other kinds of historicinformation may be useful for establishing time since contact between potentially hybridizingdata—ranging from newspaper accounts to archeological artifacts (e.g., Leak-Garcia et al. 2013).

The combination of historic information and genetic data can be used to infer the amountof gene exchange between populations. Traditional population genetic methods for estimatinggenetic differentiation, such as F statistics (Wright 1931) and their analogs, have often been usedto estimate levels of gene flow. These methods measure genetic differentiation, which can be usedto estimate historic gene flow (average number of successful immigrants per generation) but makesome potentially biologically unrealistic assumptions (Whitlock & McCauley 1999).

Recently, methods have been developed for assigning individuals into groups based on patternsof linkage equilibrium or genetic differentiation (e.g., Corander et al. 2004, Pritchard et al. 2000).Such methods can identify admixed individuals (those whose genetic content is derived from morethan one of the inferred groups). Such admixture implies recent introgression. Another method,BAYESASS (Wilson & Rannala 2003), explicitly estimates population-level introgression ratesrather than individual migrant ancestries and does not assume Hardy-Weinberg equilibrium (oftenthe case with other methods). BAYESASS and related approaches are appropriate for detectingintrogression that has occurred relatively recently (<20 generations ago). The relatively new (atleast to population genetics) method of Approximate Bayesian Computation (Beaumont 2010) hasbeen successfully applied in crop-wild introgression during domestication (e.g., Ross-Ibarra et al.2009) and has potential regarding more recent crop-WW questions. This method is versatile, given



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the range of evolutionary simulation programs now available (partially reviewed by Hoban et al.2012), with relatively few limitations for demographic scenarios that can be accommodated (e.g.,some cases of asymmetrical gene flow). However, it requires large amounts of data, especially forcomplex scenarios. Just how far into the past these methods are informative needs to be addressedwith additional simulation or controlled experimental work.

The foregoing methods are often applied to microsatellite data. As discussed above, the highmutability of microsatellites can result in homoplasy, a problem for estimating long-term intro-gression. Questions of long-term introgression measurement are better addressed with slowlyevolving markers such as SNPs or longer sequenced regions. Therefore, for longer-term intro-gression estimates, coalescent-based methods (Kuhner 2009) may be appropriate. Two widelyused programs implementing these methods are MIGRATE (Beerli 2006) and the isolationwith migration (IM) suite of programs (Hey 2010). These methods explicitly take into accountgenealogical relationships between alleles, enabling estimates of both directional introgressionrates and other demographic parameters such as divergence time and population size (Kuhner2009). MIGRATE assumes an equilibrium scenario in which population sizes and migration rateshave been stable for a long time relative to initial divergence. In contrast, the IM programsexplicitly model a nonequilibrium scenario in which divergence occurred in the relatively recentpast, allowing for the disentangling of genetic similarity due to gene flow versus shared retentionof ancestral polymorphisms. Consequently, IM is likely more appropriate for many crop-WWsystems. However, the current methods do not efficiently estimate the timing of introgression(Strasburg & Rieseberg 2011); additional relevant research would be valuable.

A “genomic clines” method for examining genomic patterns of introgression for both short- andlong-term timescales has recently been described (Gompert & Buerkle 2009, 2011). This methodcompares introgression patterns at individual loci relative to the genomic background for detectingselection that affects introgression rates. Simulations suggest that this method can be informativeon timescales as short as five generations, making it applicable to many crop-WW systems.

CROP ALLELE INTROGRESSION INTO WW POPULATIONS

Crops and their WW relatives present both disadvantages and advantages for detecting introgres-sion. Most crops began their evolutionary journey hundreds to thousands of years ago, leavinglittle time for substantial genome divergence. However, the plant improvement process may havefixed alleles that are nearly absent in WW populations because they are detrimental under naturalconditions. Undesirable traits eliminated from crops include seed dormancy, seed shattering inseed crops, and early bolting in vegetable crops (e.g., Hartman et al. 2013a, Weeden 2007). Bottle-necks from strong selection under domestication vary widely in both intensity and duration (Gross& Olsen 2010); stronger bottlenecks generally promote higher levels of differentiation betweendomesticates and their wild ancestors. Furthermore, plant breeders have occasionally introgressedwild germplasm into a crop, resulting in decreased genetic differentiation between the taxa. Suchdeliberate wild-to-crop introgression might be difficult to distinguish from introgression in theopposite direction, but methods exist for detecting asymmetric patterns of gene flow (Beerli 2006,Hey 2010). In contrast, transgenesis and mutation breeding create evolutionarily unique singlelocus (for transgenics) and multiple locus (for mutation breeding) crop-specific markers.

Crops, as part of the study system, provide numerous benefits because of their economicimportance. They are well studied; for example, at the moment, substantial genome sequencedata are available for 49 plant species, 35 of which are crops. (Michael & Jackson 2013). Dozens ofcrop species not yet sequenced have been extensively genetically mapped with much transcriptomedata available (e.g., Bowers et al. 2012). Crops have the advantage that historical and geographic



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information are often available—e.g., cultivars and the duration of cultivation in a given location.Crop genetic resources are readily available, and pure material of specific crop cultivars involvedin hybridization can be obtained for screening. Moreover, extensive germplasm collections of oldlandraces and wild accessions are available for many crops.

Consequently, reports of domesticated allele introgression into WW populations have grad-ually accumulated. Table 1 features a nonexhaustive list of crop species for which introgressionis known to have occurred or is occurring on the basis of substantial data. We provide some casestudies below as examples. These represent an array of crops from different plant families withdifferent uses, breeding systems, and life histories. None of the following crops, except for beet,have genetically engineered cultivars that are commercially available.

CASE STUDIES

Beet, Beta vulgaris vulgaris

Beet is mainly wind pollinated and predominantly outcrossing; cross pollination on the order ofa kilometer is not unusual (Bartsch 2010). Spontaneous hybridization easily occurs between thecrop B. v. vulgaris (both root beet and sugar beet) and its wild progenitor, Beta vulgaris maritima(sea beet), as documented by both descriptive and experimental studies (reviewed by Bartsch2010, Ellstrand 2003). Much of the research was first stimulated by the evolution of hybrid-derived annual weed beets in Europe’s biennial sugar beet fields. Because these bolting individualshave a woody root, their presence often substantially reduces yields and damages the machineryused in harvesting and processing sugar beets. Several studies using suites of genetically basedmorphological and molecular markers revealed that the initial weed beets were natural F1 hybridsbetween sea beets and sugar beets used as maternal parents for commercial seed production. Ifweed beets are not eradicated prior to seed dispersal, their descendants evolve quickly via crossingwith one another into stably introgressed weed beet populations well adapted to flourish in sugarbeet fields (Arnaud et al. 2010, Bartsch 2010). Weed beet seeds can survive in the soil for severalyears. Their seedlings and young plants are indistinguishable from sugar beet. They tolerate allherbicides suitable for sugar beet cropping. Thus, weed beet is hard to control without long croprotation intervals.

Introgression in the other direction (from the crop to the WW relatives) attracted attentionwhen concerns arose about the environmental risks associated with transgenic sugar beets. De-scriptive studies with molecular markers demonstrated that Italian sea beet populations growingwithin a few kilometers of beet seed multiplication sites were introgressed with domesticated alle-les. Additionally, populations of the more distantly related Beta macrocarpa in southern Californiathat were sympatric with sugar beet fields were found to have a low frequency of sugar beet–specific alleles. In both cases, about 100 years of gene flow has increased allelic diversity of theWW populations. Thus, for these taxa, crop-to-weed introgression has already delivered novelcrop alleles to WW populations and would be expected to do so for transgenes as well (Bartsch2010, Ellstrand 2003). Transgenic herbicide-resistant sugar beet is currently commercially grownin the United States ( James 2012). Without extraordinary measures to prevent gene flow, trans-gene introgression in the long run is likely if beets are grown close to compatible WW congeners,such as those found in California’s Imperial Valley.

Carrot, Daucus carota sativus

Both carrot and its WW progenitor, Daucus carota carota, are predominantly outcrossing andcapable of insect-pollinated at more than four kilometers (Rong et al. 2010). Spontaneous



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hybridization between cultivated carrot and its WW subspecies is well known to carrot indus-try seed scientists because pollen from WW relatives adjacent to cultivar seed multiplication fieldsoften contaminates commercial seed, resulting in the germination of individuals with intermediatemorphologies. Plants with similar hybrid-like morphologies have also been found in the adjacentWW populations (Wijnheimer et al. 1989).

Evidence for crop-WW introgression in this system has been demonstrated only recently.Magnussen & Hauser (2007) used amplified fragment length polymorphisms to genetically ana-lyze 71 D. c. carota plants growing within, adjacent to, and quite distant from cultivated fields. Fourindividuals (5.6%) were found to have hybrid ancestry, but did not have genotypic patterns consis-tent with F1 hybrids; they had to be F2s, backcrosses, or more advanced introgressants. They alsofound that wild carrot populations in close proximity to cultivated carrot fields were geneticallymore similar to cultivated carrot than wild carrot populations located far from cultivated carrotfields, suggesting introgression had changed the genetic structure of the WW populations.

In a three-year field experiment, Hauser & Shim (2007) showed that F1s between cultivatedand wild carrots could survive and reproduce outside cultivation, although with somewhat lowerlifetime fitness overall. Fitness varied with environment and was not so low as to impede the hybridsfrom crossing and backcrossing. The fitness of crop-wild offspring beyond first generation hybridshas not been reported yet, and the dynamics of domesticated alleles in wild carrot populationsremain unknown.

Sunflower, Helianthus annuus

Domesticated in North America, cultivated sunflower is well known to naturally hybridize withsome WW relatives (Ellstrand 2003). Wild Helianthus species are self-incompatible and insect-pollinated; the crop is self-fertile but not highly selfing. The system is well studied with regardto crop-to-WW hybridization and introgression. Numerous descriptive and experimental studiesusing a variety of molecular and morphological markers have demonstrated natural hybridizationbetween the crop and WW H. annuus as well as Helianthus petiolaris (reviewed by Ellstrand 2003).Not only has spontaneous hybridization been documented repeatedly in North America (e.g.,Linder et al. 1998), the center of Helianthus diversity, but it has also been documented in regionswhere the crop and its WW relatives have been introduced, such as Argentina (Gutierrez et al.2010).

Experimental fitness comparisons between wild H. annuus and crop-wild hybrids often showan initial F1 fitness penalty, which varies among genotypes and environments, declining oversubsequent generations of introgression (Gutierrez et al. 2011, Mercer et al. 2007, Presotto et al.2012). Such field-based experiments have also shown that some genetically based crop traits aredetrimental, whereas others, such as larger seed size, early competitive advantage, and insectresistance, have a selective advantage that could facilitate their introgression into WW populations(Baack et al. 2008, Dechaine et al. 2009, Snow et al. 2003). Interestingly, Burke & Rieseberg (2003)found that a transgene that confers disease resistance in the crop did not boost the fitness of crop ×WW hybrids in the presence of the disease organism.

Several studies have demonstrated that cultivated sunflower alleles have introgressed into WWpopulations. For example, in a five-year study, Whitton et al. (1997) found crop-specific randomamplified polymorphic DNA markers continuing to spread spatially through a WW H. annuuspopulation after a single season of natural hybridization with an adjacent crop population. Likewise,interspecific introgression, in particular into H. petiolaris, has been documented with suites ofmolecular markers (e.g., Gutierrez et al. 2010, Rieseberg et al. 1999). With known significantintrogression into WW populations, so much fitness data, and the development of resources



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(such as the 10,000 locus Helianthus genetic map) (Bowers et al. 2012), Helianthus is an importantmodel system for studying how introgression proceeds and the role it plays in evolution.

Lettuce, Lactuca sativa

Although both cultivated lettuce and the cross-compatible WW Lactuca serriola are predominantlyselfing, substantial cross-pollination by insects has been observed. Given those data, the factthat both species often grow in close proximity, and their flowering periods overlap for weeks,experimental field studies have been conducted to measure crop-to-WW outcrossing rates. Typicalhybridization rates are on the order of a few percent or less, but individual plants at short distancemay bear as much as 26% hybrid seed (D’Andrea et al. 2009).

Uwimana et al. (2012a) genotyped a wide range of Lactuca germplasm with ten microsatelliteloci: almost eight thousand individual samples of European L. sativa and L. serriola accessions.Genetic admixture analysis by the model-based clustering program structure (Pritchard et al.2000) revealed two clusters and 7% introgressed plants. The Bayesian NewHybrids program(Anderson & Thompson 2002) revealed that some of these introgressants were the result of severalgenerations of selfing from either an F1 or a BC1 ancestor, consistent with how introgressionshould proceed for such highly selfing plants.

Hooftman and coworkers conducted a set of field experiments to assess the effects of hybridancestry on fitness in Lactuca allowing for natural levels of competition. They found hybrid-derived plants (via both selfing and backcrossing) outperforming the parental species for at leastup to four generations (Hooftman et al. 2007). Second generation hybrids, when compared to thewild relative, had twice the relative fitness based on seed-output-per-seed-sown. Differences ingermination and survivorship were found for all generations; differences in seed output were foundonly for the first two generations (Hooftman et al. 2005). The degree of the fitness differencedecreased over generations (Hooftman et al. 2007), suggesting heterosis was the underlying causeof the initial fitness increase. However, transgressive segregation occurred as well: Several of theselfing lines continuously outperformed the wild species in a subsequent experiment (Hooftmanet al. 2009). Such patterns suggest the possible evolution of adaptive gene combinations. Thesame pattern was shown independently by Hartman et al. (2013b) using recombinant inbred linesfrom a cross between a Californian L. serriola accession and a L. sativa cultivar.

Molecular evidence also supports the hypothesis that selfing has fixed Lactuca adaptive trans-gressive gene combinations. Using those plants that survived until flowering from their multiyearstudy, Hooftman et al. (2011) demonstrated that various parts of the hybrid-derived genomes wereskewed toward one or the other parent species. Quantitative trait loci studies using the best per-forming lines revealed an adaptive combination of alleles from both parental species at unlinkedgenomic locations. Similar results were obtained from other recent studies of lines derived fromL. sativa × L. serriola hybrids (Hartman et al. 2012, 2013b; Uwimana et al. 2012b).

Asian Cultivated Rice, Oryza sativa sativa

Despite the crop’s high selfing rate, natural hybridization occurs between cultivated rice and twoof its close WW relatives. One is cultivated rice’s ancestor, Oryza rufipogon. The other is weedyrice; Oryza sativa f. spontanea, a noxious weed of cultivated rice that closely mimics the crop butdisperses (shatters) its seed before it can be harvested. These taxa are also self-compatible but oftenhave somewhat higher outcrossing rates than cultivated rice (Andersson & de Vicente 2010, Lu& Snow 2005). Like most grasses, wind disperses pollen in domesticated rice and its WW rela-tives. Support for spontaneous hybridization comes from descriptive studies, showing that hybrid



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swarms typically occur when the crop grows in close proximity with its WW relatives. Likewise,experimental field studies have measured crop-to-WW Oryza hybridization rates. At multime-ter distances, hybridization rates are typically <1% (Lu & Yang 2009). However, hybridizationrates between intermixed cultivated rice and O. rufipogon can be as high as 18% (Wang et al.2006).

Studies employing molecular markers have demonstrated crop allele introgression into somesympatric and peripatric populations of both the wild ancestor and the derived weedy rice (dos ReisGoulart et al. 2012, Song et al. 2006, Xia et al. 2011). Furthermore, genetic analysis has confirmedsome weedy rice populations are descendants of O. sativa × O. rufipogon hybrids, whereas othersevolved directly from feral cultivated rice (Ellstrand et al. 2010).

Numerous studies have evaluated the relative fitness of Oryza hybrids and introgressants, withand without transgenes. Given the polygenic dominance of shattering, direct comparison of hy-brids with their WW parents in a common garden experiment is reasonable. F1s of the crop andwild rice generally have some reduced sexual fitness relative to the pure wild parent, but haveincreased vegetative tillering (Song et al. 2004); however, F1s of the crop and weedy rice generallyhave about the same fitness as pure weedy rice (Lu & Yang 2009).

Lu and colleagues have started conducting experimental studies to examine the fitness con-sequences of transgenic insect resistance introgressed into WW rice. Their limited research onhybrids between cultivated rice and O. rufipogon and their descendants has shown little significanteffect of the crop transgene (either Bt or CpTI, which confer resistance to insects) on seed dor-mancy (Dong et al. 2011). With regard to hybrids and introgressants involving weedy rice, theresults have been complex, depending on both genotype and environment. Transgenic hybridsand progeny through F3 showed increased fecundity under natural insect pressure compared withtheir nontransgenic counterparts, but the fecundity boost disappeared under low insect pressure(Yang et al. 2011). Transgenic rice has been deregulated in the United States, Iran, and Chinabut, to our knowledge, it is has not yet been grown commercially anywhere.

Nonetheless, considering the results of the following recent study, favorable transgenes arelikely to flow easily from rice into nearby populations of weedy rice, as well as into populationsof the wild ancestor, where crop gene flow has already been shown to occur at higher rates thanin the weed. Nontransgenic rice with herbicide resistance to imidazolinone has been introducedworldwide. The evolution of imidazolinone resistance in companion weedy rice populations hasoccurred in several locations. But was the evolution of resistance due to spontaneous mutationor introgression of the resistance allele from the crop? Researchers in Brazil (dos Reis Goulartet al. 2012) addressed this question by genetically analyzing thousands of resistant weedy riceplants from dozens of populations for the resistance mutations known in the cultivars as well as forcultivar-specific microsatellite markers. Almost 99% of the herbicide-resistant plants had evolvedvia introgression of the crop allele; 1.1% evolved as a result of spontaneous mutation.

Radish, Raphanus sativus

Radish and its WW relative Raphanus raphanistrum are insect-pollinated and self-incompatibleannuals (Snow & Campbell 2005). The two often naturally hybridize where these taxa co-occur,often forming localized hybrid swarms (Stace 1975). Surprisingly, these spontaneous hybrid pop-ulations have attracted little attention from researchers (Snow & Campbell 2005).

However, in California, bilateral introgression between the two introduced taxa over the pastcentury or so has been so extensive that the species have coalesced into a widespread set of hybrid-derived populations, as demonstrated by both morphological and molecular analyses (reviewed byEllstrand et al. 2010). Although nowhere near as problematic as Europe’s weed beet, “California



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wild radish” is classified as both an agricultural weed and as an invasive plant. The molecularevidence supports a history of multiple hybridizations that have contributed to what is now amore-or-less evolutionary cohesive metapopulation.

Despite the relative uniformity of molecular markers, field experiments have shown that thesenew populations have undergone adaptive evolution (reviewed by Ellstrand et al. 2010). Californiawild radish has a higher maternal fitness than either progenitor in several environments, producingmore seeds per plant. Another set of reciprocal transplant experiments involving plants fromnorthern and southern sites revealed the evolution of local adaptation in probably less than 100generations.

Snow and colleagues (Campbell et al. 2006, Hovick et al. 2012, Snow et al. 2010) conductedsome long-term experiments to study the consequences of crop allele introgression into natu-ralized populations of R. raphanistrum. Under natural field conditions, they created four pureR. raphanistrum populations and four populations of crop-WW F1s, allowing these annual plantsto evolve in Michigan year by year. They harvested seeds from both types of populations after fourgenerations, planting them in common gardens to compare fitness. Fitness of the hybrid-derivedlineages had recovered in a couple of generations. The advanced generation hybrids had slightlylower lifetime fecundity in than WW R. raphanistrum. But when Michigan-evolved hybrids andtheir WW parent were grown in southern Californian (Campbell et al. 2006) and southeasternTexan (Hovick et al. 2012) common gardens, the fecundity of the hybrid lineage was roughlytriple that of the pure parent.

The Michigan field experiment allowed for monitoring of some unlinked crop-specific allelesfor several years. Each showed a different evolutionary trajectory over ten years. One crop trait, adominant white flower color allele, was found to be present 14 years after the experiment’s start(Snow et al. 2010).

THEORETICAL STUDIES

Motivated by recent interest in transgene flow, computational and mathematical research onintrogression has been steadily increasing. Such theoretical studies on transgene introgression arealso usually valid for general crop allele introgression. Typically, theoretical studies concern twotopics: identifying factors affecting introgression and transgenic plant risk assessment. Sometimesthe same analysis considers both, but it is instructive to address them separately.

Factors Affecting Introgression

The dynamics of introgression depend on evolutionary (e.g., gene flow, positive selection, neg-ative selection, variation in selection), spatial (e.g., metapopulation structure), ecological (e.g.,environment-dependent heterosis), genetic (e.g., ploidy differences between hybridizing taxa,linkage of the allele in question to other genes under selection), and many other factors. Space lim-itations preclude an exhaustive discussion of them here. Theoretical approaches have proven wellsuited for evaluating the relative importance of what are considered some of the most importantfactors.

General population genetics theory provides some simple predictions for introgression (sum-marized by Ellstrand 2003). If a modest amount of gene flow occurs without opposing selection(e.g., one successful immigrant per generation), immigrant alleles can establish in the recipientpopulation. Under opposing selection, the immigrant alleles eventually disappear after a singlegene flow incident. But if gene flow reoccurs uniformly over generations with opposing selection,immigrant alleles may be maintained in immigration-selection equilibrium.



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Selection pressure and gene flow often vary in space and time. Kuparinen & Schurr (2007)developed a spatially explicit model allowing for spatial and temporal heterogeneities in gene flowand selection. Furthermore, they considered different modes of crop transgene deployment andexpression. They found that if crop plants are heterozygous for a transgene enhancing the fitness ofWW relatives, introgression levels would be reduced because, compared with homozygous plants,immigration pressure is halved. Using recessive transgenes (as opposed to dominant or additiveones) limits the spread of advantageous genes because positive selection on recessive genes is veryweak when they are rare. Therefore, unless gene flow is high and/or recurrent, recessive transgenesare more likely to be lost through genetic drift.

A transgene’s location in the genome can also affect introgression. Inserting a transgene next toa domestication gene that is deleterious in WW populations could drastically reduce introgression(Gressel 1999, Stewart et al. 2003). Ghosh et al. (2012a) used both mathematical and simulationapproaches to address mitigation by linkage and found it to be a viable mitigation strategy. Theydefined the hazard rate as the probability per unit time that a permanent transgene lineage isformed, given that one has not previously been formed. Small WW populations yielded highhazard rates owing to the force of repeated hybridization. At intermediate population sizes, hazardrates were lower because low-frequency crop alleles are removed by drift. Selection was only ofimportance in large populations, leading to an increase in the hazard rate.

A WW population receives most gene flow from a crop when intermixed with or adjacentto the crop field. Such WW populations are rarely isolated, but often form a part of a largerWW metapopulation. Some recent models examined introgression in the context of gene flowbetween metapopulation demes in concert with their extinction-recolonization. One such study(Meirmans et al. 2008) found that even very low levels of introgression and positive selection canlead to the fixation of a transgene in the metapopulation. Thus, metapopulation dynamics can playan important role in promoting introgression.

Quantitative Risk Assessment

Traditional transgene risk assessment defines risk as a function of exposure and hazard (“an act orphenomenon that has the potential to produce harm or other undesirable consequences to humansor what they value”; Natl. Res. Counc. 1996, p. 215), and transgene introgression involves the“exposure” component of that analysis. (Note the definition of hazard is quite different than the“hazard rate” defined earlier in this article.) Introgression is not a hazard in itself. Introgressionhas been largely associated with possible environmental hazards of depletion of genetic diversity,evolution of weediness/invasiveness, and extinction. The hazards of introgression are not neces-sarily restricted to environmental effects (de Jong & Rong 2013). For example, introgression hasbeen associated with various culturally identified hazards, such as the altered genetic makeup ofwild plants sacred to indigenous peoples (e.g., Walker & Doerfler 2009). These hazards are notunique to transgene flow but could be associated with gene flow from any crop to a wild relativeand have, on rare occasion, occurred (Ellstrand 2003).

Therefore, a complete transgene introgression risk assessment should comprise the probabil-ity of introgression as well as its consequences (Hill 2005) and how it differs from nontransgenicintrogression. Although natural crop-WW hybridization occurs for most crop-WW relative com-binations, it is generally rare (Ellstrand 2003, Stewart et al. 2003). Given a low hybridization rateand the fact that the size and lifespans of recipient WW populations are often limited, the intro-gression process is inherently strongly stochastic, driven by both selection and genetic drift. Thus,stochastic approaches can supplement the long-used deterministic models to investigate gene flowand introgression in transgene risk assessment.



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Classical population genetic models (e.g., Kimura 1962) predict that drift often prevents thefixation of a gene that is present in only a few copies in a large population, even if it is selectivelyfavored. Consequently, if hybridization occurs once and at a low rate, introgression may fail tooccur. However, a crop is rarely grown once near a WW population; repeated crop-to-WW geneflow typically occurs. If gene flow is recurrent and the interaction period is indefinitely long, thenintrogression is inevitable for neutral and favorable alleles.

Inevitable does not necessarily mean immediate, because it might still take a very long timebefore an allele permanently invades a population. Whether a transgene goes to fixation afterhybridization stops at a given time is relevant to risk assessment, because fixation represents apermanent evolutionary change to the population in question. Ghosh & Haccou (2010) quantifiedthe risk of introgression based on probability of fixation, proposing that the hazard rate (definedabove) is a suitable measure of the exposure component of risk. Ghosh et al. (2012b) consideredeffects of periodical or permanent interruption of gene flow on the hazard rate.

Because the introgression process depends on so many factors, a comprehensive model to quan-tify introgression probability will necessarily be detailed, especially when considering a metapop-ulation scale. Wilkinson et al. (2003) did this by using a combination of modeling and remotesensing to estimate the number of Brassica rapa × Brassica napus hybrids formed annually acrossthe entire United Kingdom. DiFazio et al. (2012) used a smaller spatial scale, basing their modelon extensive geographic information systems information including topology and land-use infor-mation. They parameterized their model using data from experiments with their model system,Populus trichocarpa, measuring pollen flow, seed flow, and establishment. With this model, theywere able to generate very detailed predictions about transgene escape. However, as a consequenceof the size of the model, it could only be run for a limited number of generations and thereforecould not be used to quantify the eventual fate of the transgene in the population, that is, itsprobability of reaching fixation. Such detailed models are large, with long run times. With somany different parameters, each of which has to be estimated from experimental or observationaldata, quantifying the uncertainty in the estimated risks is challenging. Such “uncertainty aboutuncertainty” makes interpretation of the results difficult, especially for those policy makers whodesire clear-cut answers. Of course, sensitivity analyses can help to distinguish which parametersare important versus those that contribute little to the results. Furthermore, limiting a model to asmall set of realistic usage scenarios often makes an exhaustive exploration of the parameter spaceunnecessary. DiFazio et al. (2012) combined these two approaches and compared two differentusage scenarios: a small field trial and a large-scale plantation. Their sensitivity analyses indicatedthat increasing seed and vegetative dispersal led to a proportionally larger increase in transgenespread in the plantation than in the field trial. Their results suggest that at different scales ofemployment of the transgenic crop, different factors determine the introgression risk.

CONCLUSIONS AND A LOOK TO THE FUTURE

A few decades ago the idea of crops spontaneously hybridizing with their WW relatives was contro-versial. That question was resolved with data demonstrating that such spontaneous hybridizationoccurs for most crops, but with varying frequency depending on many factors. The controversyhas since moved on to the extent and nature of crop allele introgression into WW populations andits consequences. As discussed above, techniques for collecting data and analyzing data regardingintrogression have made a quantum leap since the last major review of introgression in plants twodecades ago (Rieseberg & Wendel 1993).

Consequently, strong evidence of spontaneous crop allele introgression has now been foundfor several crops (Table 1). Our case studies reveal that such evidence varies over crops. For some



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cases, simply the fact of introgression is known, in others, it is clear that introgression played a keyrole in the evolution of a more problematic weed (e.g., transfer of herbicide resistance in rice) oreven a new noxious weed (e.g., weed beet). Experimental work on post-F1 fitness with and withouttransgenes is becoming more common. Such data will provide information to feed emergingdemographic models for predicting the progress of introgression (e.g., see lettuce case study).

Although concern about transgene introgression fueled interest in crop-to-WW introgressionresearch and millions of hectares have been planted to transgenic crops for more than two decades,examples of spontaneous transgenic crop × WW hybridization remain exceedingly few. Thediscovery of transgenes in WW populations is primarily due to the fact that the vast bulk oftransgenic commercial plants are restricted to four crop species (maize, cotton, oilseed rape, andsoybean) generally planted far (hundreds of kilometers) from their WW relatives.

Transgenes in WW populations are known from three cases. Evidence for hybridization isclear for those cases but not for introgression.

Transgenic oilseed rape has been commercially grown in Quebec, where it overlaps withthe range of the closely related weed, Brassica rapa. Transgenic hybrids were identified in twoseparate locations in Quebec. The oilseed rape crop is no longer grown at either site. TheBrassica populations have been monitored, and the transgene appears to be decreasing in fre-quency. One individual was identified as a putative introgressant based on joint cytogenetic dataand the herbicide-resistance phenotype of the transgene (Warwick et al. 2008).

In 2003, pollen and seed unintentionally dispersed from field trials of transgenic cultivatedcreeping bentgrass. Transgenic F1s with weedy creeping bentgrass were subsequently identified.No post-F1 transgenic bentgrass has yet been reported, but it is not clear that such plants have yetbeen sought (Reichman et al. 2006).

Transgenic cotton is grown in Mexico, but generally far from WW cotton populations. How-ever, after removing the lint, cottonseed is transported in open vehicles throughout the country.Wegier et al. (2011) used immunoassays to test for transgenes in cotton seeds collected fromapparently wild Mexican Gossypium hirsutum plants, that is, those whose morphology showed noevidence of introgression from the crop. F1s are typically intermediate to the crop and the wildplant. Plants from four populations tested positive for one or more transgenes, suggesting theyare not hybrids but introgressants. More study of all three systems would be helpful to establishwhether introgression is occurring or the establishment of transgenes is transient.

The study of introgression of crop alleles into WW populations is a healthy field. It has comea long way and is poised for future growth to answer both basic and applied questions at theinterface of evolution and ecology. Although interest in crop-WW gene flow is rapidly growing,the same can be said for studies of plant introgression that do not involve crops at all. Spacelimitations preclude a comprehensive review. But some approaches do not involve expensive andsophisticated genetic analysis. For example, Helianthus annuus ssp. texanus is a hybrid derivativeof wild H. annuus ssp. annuus and H. debilis. By growing these three taxa, synthetic hybrids,and backcrosses in two common gardens and measuring how fitness covaried with herbivorepressure and ecophysiological, phenological, and architectural traits, Whitney and colleagues(2006, 2010) were able to identify the adaptive introgression of specific abiotic and biotic traitsfrom H. debilis into H. annuus. Obviously, such research can be a model for future work on crop-WW introgression.

DISCLOSURE STATEMENT

The authors are not aware of any affiliations, memberships, funding, or financial holdings thatmight be perceived as affecting the objectivity of this review.



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ACKNOWLEDGMENTS

This work resulted from the workshop “Transgenes going wild?” held in July 2011 (http://www.lorentzcenter.nl/lc/web/2011/451/info.php3?wsid=451). We thank the Lorentz Center ofLeiden University for hosting the workshop. The workshop was supported by the NetherlandsOrganization for Scientific Research (NWO) and the research program “Ecology RegardingGenetically Modified Organisms” (ERGO). The authors thank the following for their support:the John Simon Guggenheim Memorial Fellowship Foundation, the National Science Founda-tion (DEB 1020799) the United States Department of Agriculture Biotechnology Risk Assess-ment Grants program (2010-39211-21699, 2008-33522-04856), National Science Foundation ofChina (31271683), and the ERGO program (838.06.031, 838.06.041, 838.06.042, 838.06.110,838.06.112).

LITERATURE CITED

Anderson E. 1949. Introgressive Hybridization. New York: WileyAnderson EC, Thompson EA. 2002. A model-based method for identifying species hybrids using multilocus

data. Genetics 160:1217–29Andersson MS, de Vicente MC. 2010. Gene Flow between Crops and Their Wild Relatives. Baltimore: Johns

Hopkins Univ. PressArnaud JF, Fenart S, Cordellier M, Cuguen J. 2010. Populations of weedy crop-wild hybrid beets show

contrasting variation in mating system and population genetic structure. Evol. Appl. 3:305–18Arnold M. 2006. Evolution through Genetic Exchange. Oxford, UK: Oxford Univ. PressBaack EJ, Rieseberg LH. 2007. A genomic view of introgression and hybrid speciation. Curr. Opin. Genet.

Dev. 17:513–18Baack EJ, Sapir Y, Chapman MA, Burke JM, Rieseberg LH. 2008. Selection on domestication traits and

quantitative trait loci in crop-wild sunflower hybrids. Mol. Ecol. 17:666–77Baird NA, Etter PD, Atwood TS, Currey MC, Shiver AL, et al. 2008. Rapid SNP discovery and genetic

mapping using sequenced RAD markers. PLoS One 3(10):e3376Bartsch D. 2010. Gene flow in sugar beet. Sugar Tech. 12:201–6Beaumont MA. 2010. Approximate Bayesian computation in evolution and ecology. Annu. Rev. Ecol. Evol. Syst.

41:379–406Beerli P. 2006. Comparison of Bayesian and maximum-likelihood inference of population genetic parameters.

Bioinformatics 22:341–45Bowers JE, Bachlava E, Brunick RL, Rieseberg LH, Knapp SJ, Burke JM. 2012. Development of a 10,000

locus genetic map of the sunflower genome based on multiple crosses. G3 2:721–29Buerkle C, Morris R, Asmussen M, Rieseberg L. 2000. The likelihood of homoploid hybrid speciation. Heredity

84:441–51Burke JM, Rieseberg LH. 2003. Fitness effects of transgenic disease resistance in sunflowers. Science 300:1250Campbell LG, Snow AA, Ridley CE. 2006. Weed evolution after crop gene introgression: greater survival and

fecundity of hybrids in a new environment. Ecol. Lett. 11:1198–209Corander J, Waldmann P, Marttinen P, Sillanpaa MJ. 2004. BAPS 2: enhanced possibilities for the analysis

of genetic population structure. Bioinformatics 20:2363–69D’Andrea L, Felber F, Guadagnuolo R. 2009. Hybridization rates between lettuce (Lactuca sativa) and its wild

relative (L. serriola) under field conditions. Environ. Biosaf. Res. 7:61–71Dechaine JM, Burger JC, Chapman MA, Seiler GJ, Brunick R, et al. 2009. Fitness effects and genetic archi-

tecture of plant–herbivore interactions in sunflower crop–wild hybrids. New Phytol. 184:828–41de Jong TJ, Rong J. 2013. Crop to wild gene flow: Does more spohisticated research provide better risk

assessment? Environ. Sci. Policy 27:135–40den Nijs HCM, Bartsch D, Sweet J. 2004. Introgression from Genetically Modified Plants into Wild Relatives.

Wallingford: CABI



Ann

u. R

ev. E

col.

Evo

l. Sy

st. 2

013.

44. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsite

it va

n A

mst

erda

m o

n 10

/10/

13. F

or p

erso

nal u

se o

nly.

http://www.lorentzcenter.nl/lc/web/2011/451/info.php3?wsid=451

http://www.lorentzcenter.nl/lc/web/2011/451/info.php3?wsid=451


deWet JMJ, Harlan JR. 1975. Weeds and domesticates: evolution in the man-made habitat. Econ. Bot. 29:99–108

Difazio S, Leonardi S, Slavov G, Garman S, Adams W, Strauss S. 2012. Gene flow and simulation of transgenedispersal from hybrid poplar plantations. New Phytol. 193:903–15

Dong SS, Xiao MQ, Rong J, Liao H, Lu B-R, et al. 2011. No effect of transgene and strong wild parent effectson seed dormancy in crop-wild hybrids of rice: implications for transgene persistence in wild populations.Ann. Appl. Biol. 159:348–57

dos Reis Goulart ICG, Pacheco MT, Nunes AL, Merotto A. 2012. Identification of origin and analysis ofpopulation structure of field-selected imidazolinone-herbicide resistant red rice (Oryza sativa). Euphytica187:437–47

Ellstrand NC. 2003. Dangerous Liaisons? When Cultivated Plants Mate with Their Wild Relatives. Baltimore:Johns Hopkins Univ. Press

Ellstrand NC, Elam DR. 1993. Population genetic consequences of small population size: implications forplant conservation. Annu. Rev. Ecol. Syst. 24:217–42

Ellstrand NC, Heredia SM, Leak-Garcia JA, Heraty JM, Burger JC, et al. 2010. Crops gone wild: evolutionof weeds and invasives from domesticated ancestors. Evol. Appl. 3:494–504

Elshire RJ, Glaubitz JC, Sun Q, Poland JA, Kawamoto K, et al. 2011. A robust, simple genotyping-by-sequencing (GBS) approach for high diversity species. PLoS ONE 6(5):e19379

Ghosh A, Haccou P. 2010. Quantifying stochastic introgression processes with hazard rates. Theor. Popul. Biol.77:171–80

Ghosh A, Meirmans P, Haccou P. 2012a. Quantifying introgression risk with realistic population genetics.Proc. R. Soc. B 279:4747–54

Ghosh A, Serra MC, Haccou P. 2012b. Quantifying time-inhomogeneous stochastic introgression processeswith hazard rates. Theor. Popul. Biol. 81:253–63

Gompert Z, Buerkle CA. 2009. A powerful regression-based method for admixture mapping of isolation acrossthe genome of hybrids. Mol. Ecol. 18:1207–24

Gompert Z, Buerkle CA. 2011. Bayesian estimation of genomic clines. Mol. Ecol. 20:2111–27Green RE, Krause J, Briggs AW, Maricic T, Stenzel U, et al. 2010. A draft sequence of the Neandertal genome.

Science 328:710–22Gressel J. 1999. Tandem constructs: preventing the rise of superweeds. Trends Biotechnol. 17:361–66Gross BL, Olsen KM. 2010. Genetic perspectives on crop domestication. Trends Plant Sci. 15:529–37Gutierrez A, Cantamutto M, Poverene M. 2011. Persistence of sunflower crop traits and fitness in Helianthus

petiolaris populations. Plant Biol. 13:821–30Gutierrez A, Carrera A, Basualdo J, Rodriguez R, Cantamutto M, Poverene M. 2010. Gene flow between

cultivated sunflower and Helianthus petiolaris (Asteraceae). Euphytica 172:67–76Hajjar R, Hodgkin T. 2007. The use of wild relatives in crop improvement: a survey of developments over

the last 20 years. Euphytica 156:1–13Hartman Y, Hooftman DAP, Schranz ME, van Tienderen PH. 2013a. QTL analysis reveals the genetic

architecture of domestication traits in Crisphead lettuce. Genet. Resour. Crop Evol. 60:1487–500Hartman Y, Hooftman DAP, Uwimana B, van de Wiel CCM, Smulders MJM, et al. 2012. Genomic regions

in crop-wild hybrids of lettuce are affected differently in different environments: implications for cropbreeding. Evol. Appl. 5:629–40

Hartman Y, Uwimana B, Hooftman DAP, Schranz ME, van de Wiel CCM, et al. 2013b. Genomic andenvironmental selection patterns in two distinct lettuce crop-wild hybrid crosses. Evol. Appl. 6:569–84

Hauser TP, Shim SI. 2007. Survival and flowering of hybrids between cultivated and wild carrots (Daucuscarota) in Danish grasslands. Environ. Biosaf. Res. 6:237–47

Hey J. 2010. Isolation with migration models for more than two populations. Mol. Biol. Evol. 27:905–20Hill R. 2005. Conceptualizing risk assessment methodology for genetically modified organisms. Environ.

Biosaf. Res. 4:67–70Hoban S, Bertorelle G, Gaggiotti OE. 2012. Computer simulations: tools for population and evolutionary

genetics. Nat. Rev. Genet. 13:110–22



Ann

u. R

ev. E

col.

Evo

l. Sy

st. 2

013.

44. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsite

it va

n A

mst

erda

m o

n 10

/10/

13. F

or p

erso

nal u

se o

nly.


Hooftman DAP, Flavell AJ, Jansen H, den Nijs JCM, Syed NH, et al. 2011. Locus-dependent selection incrop-wild hybrids of lettuce under field conditions and its implication for GM crop development. Evol.Appl. 4:648–59

Hooftman DAP, Hartman Y, Oostermeijer JGB, den Nijs JCM. 2009. Existence of vigorous lineages ofcrop-wild hybrids in lettuce under field conditions. Environ. Biosaf. Res. 8:203–17

Hooftman DAP, Jong MJD, Oostermeijer JGB, den Nijs JCM. 2007. Modelling the long-term consequencesof crop-wild relative hybridization: a case study using four generations of hybrids. J. Appl. Ecol. 44:1035–45

Hooftman DAP, Oostermeijer JGB, Jacobs MMJ, den Nijs JCM. 2005. Demographic vital rates determinethe performance advantage of crop-wild hybrids in lettuce. J. Appl. Ecol. 42:1086–95

Hovick SM, Campbell LG, Snow AA, Whitney KD. 2012. Hybridization alters early life-history traits andincreases plant colonization success in a novel region. Am. Nat. 179:192–203

James C. 2012. Global status of commercialized biotech/GM crops: 2011. ISAAA Brief No. 43. Ithaca, NY:ISAAA

Jarvis DJ, Hodgkin T. 1999. Wild relatives and crop cultivars: detecting natural introgression and farmerselection of new genetic combinations in agroecosystems. Mol. Ecol. 8:S159–73

Kiaer MP, Philipp M, Jørgensen RB, Hauser TP. 2007. Genealogy, morphology and fitness of spontaneoushybrids between wild and cultivated chicory (Cichorium intybus). Heredity 99:112–20

Kimura M. 1962. On the probability of fixation of mutant genes in a population. Genetics 47:713–19Kovalic D, Garnaat C, Guo L, Yan Y, Groat J, et al. 2012. The use of next generation sequencing and junction

sequence analysis bioinformatics to achieve molecular characterization of crops improved through modernbiotechnology. Plant Genome 5:149–63

Kuhner MK. 2009. Coalescent genealogy samplers: windows into population history. Trends Ecol. Evol. 24:86–93

Kuparinen A, Schurr FM. 2007. A flexible modelling framework linking the spatio-temporal dynamics of plantgenotypes and populations: application to gene flow from transgenic forests. Ecol. Model. 202:476–86

Kwit C, Moon HS, Warwick SI, Stewart CN Jr. 2011. Transgene introgression in crop relatives: molecularevidence and mitigation strategies. Trends Biotechnol. 29:284–93

Ladizinsky G. 1985. Founder effect in crop-plant evolution. Econ. Bot. 39:191–99Leak-Garcia JA, Holt JS, Kim S-C, Mu L, Mejıas JA, Ellstrand NC. 2013. More than multiple introductions:

multiple taxa contribute to the genesis of the invasive California’s wild artichoke thistle. J. Syst. Evol.51:295–307

Levin DA, Francisco-Ortega J, Jansen RK. 1996. Hybridization and the extinction of rare plant species.Conserv. Biol. 10:10–16

Linder CR, Taha I, Seiler GJ, Snow AA, Rieseberg LH. 1998. Long-term introgression of crop genes intowild sunflower populations. Theor. Appl. Genet. 96:339–47

Lu B-R, Snow AA. 2005. Gene flow from genetically modified rice and its environmental consequences.BioScience 55:669–78

Lu B-R, Yang C. 2009. Gene flow from genetically modified rice to its wild relatives: assessing potentialecological consequences. Biotechnol. Adv. 27:1083–91

Magnussen LS, Hauser TP. 2007. Hybrids between cultivated and wild carrots in natural populations inDenmark. Heredity 99:185–92

Maughan PJ, Smith SM, Fairbanks, DJ, Jellen EN. 2011. Development, characterization, and linkage mappingof single nucleotide polymorphisms in the grain amaranths (Amaranthus sp.). Plant Genome 4:92–101

Meirmans PG, Bousquet J, Isabel N. 2008. A metapopulation model for the introgression from geneticallymodified plants into their wild relatives. Evol. Appl. 2:160–71

Mercer KL, Andow DA, Wyse DL, Shaw RG. 2007. Stress and domestication traits increase the relative fitnessof crop-wild hybrids in sunflower. Ecol. Lett. 10:383–93

Michael TP, Jackson S. 2013. The first 50 plant genomes. Plant Genome 6(2): In press (doi:10.3835/plantgenome2013.03.0001in)

Natl. Res. Counc. 1996. Understanding Risk: Informing Decisions in a Scientific Society. Washington, DC: Natl.Acad. Press

Natl. Res. Counc. 2002. Environmental Effects of Transgenic Plants. Washington, DC: Natl. Acad. Press



Ann

u. R

ev. E

col.

Evo

l. Sy

st. 2

013.

44. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsite

it va

n A

mst

erda

m o

n 10

/10/

13. F

or p

erso

nal u

se o

nly.


Organ. Econ. Co-op. Dev. 1993. Safety Considerations for Biotechnology: Scale-up of Crop Plants. Paris: OECDPresotto A, Ureta MS, Cantamutto M, Poverene M. 2012. Effects of gene flow from IMI resistant sunflower

crop to wild Helianthus annuus populations. Agric. Ecosyst. Environ. 146:153–61Pritchard JK, Stephens M, Donnelly P. 2000. Inference of population structure using multilocus genotype

data. Genetics 155:945–59Reichman JR, Watrud LS, Lee EH, Burdick CA, Bollman MA, et al. 2006. Establishment of transgenic

herbicide-resistant creeping bentgrass (Agrostis stolonifera L.) in nonagronomic habitats. Mol. Ecol.15:4243–55

Rieseberg LH, Ellstrand NC. 1993. What can molecular and morphological markers tell us about planthybridization? Crit. Rev. Plant Sci. 12:213–41

Rieseberg LH, Kim MJ, Seiler GJ. 1999. Introgression between the cultivated sunflower and a sympatric wildrelative, Helianthus petiolaris (Asteraceae). Int. J. Plant Sci. 160:102–8

Rieseberg LH, Wendel JF. 1993. Introgression and its consequences in plants. In Hybrid Zones and the Evolu-tionary Process, ed. R Harrison, pp. 70–109. New York: Oxford Univ. Press

Rong J, Janson S, Umehara M, Ono M, Vrieling K. 2010. Historical and contemporary gene dispersal in wildcarrot (Daucus carota ssp. carota) populations. Ann. Bot. 106:285–96

Ross-Ibarra J, Tenaillon M, Gaut BS. 2009. Historical divergence and gene flow in the genus Zea. Genetics181:1399–413

Schierenbeck KA, Ellstrand NC. 2009. Hybridization and the evolution of invasiveness in plants and otherorganisms. Biol. Invasions 11:1093–105

Snow AA, Campbell LG. 2005. Can feral radishes become weeds? In Crop Ferality and Volunteerism, ed. JGressel, pp. 193–208. Boca Raton, FL: CRC Press

Snow AA, Culley TM, Campbell LG, Sweeney PM, Hegde SG, Ellstrand NC. 2010. Long-persistence ofcrop alleles in weedy populations of wild radish (Raphanus raphanistrum). New Phytol. 186:537–48

Snow AA, Pilson D, Rieseberg LH, Paulsen MJ, Pleskac N, et al. 2003. A Bt transgene reduces herbivory andenhances fecundity in wild sunflowers. Ecol. Appl. 13:279–86

Song ZP, Lu B-R, Wang B, Chen JK. 2004. Fitness estimation through performance comparison of F1 hybridswith their parental species Oryza rufipogon and O. sativa. Ann. Bot. 93:311–16

Song ZP, Zhu WY, Rong J, Lu B-R. 2006. Evidences of introgression from cultivated rice to Oryza rufi-pogon (Poaceae) populations based on SSR fingerprinting: implications for wild rice differentiation andconservation. Evol. Ecol. 20:501–22

Stace CA. 1975. Hybridization and the Flora of the British Isles. London: AcademicStace CA. 2010. New Flora of the British Isles. Cambridge, UK: Cambridge Univ. Press. 3rd ed.Stewart CN Jr, Halfhill MD, Warwick SI. 2003. Transgene introgression from genetically modified crops to

their wild relatives. Nat. Rev. Genet. 4:806–17Strasburg JL, Rieseberg LH. 2011. Interpreting the estimated timing of migration events between hybridizing

species. Mol. Ecol. 20:2353–66Twyford AD, Ennos RA. 2012. Next-generation hybridization and introgression. Heredity 108:179–89Uwimana B, D’Andrea L, Felber F, Hooftman DAP, den Nijs HCM, et al. 2012a. A Bayesian analysis of gene

flow from crops to their wild relatives: cultivated (Lactuca sativa L.) and prickly lettuce (L. serriola L.), andthe recent expansion of L. serriola in Europe. Mol. Ecol. 21:2640–54

Uwimana B, Smulders MJM, Hooftman DAP, Hartman Y, van Tienderen PH, et al. 2012b. Crop to wildintrogression in lettuce: following the fate of crop genome segments in backcross populations. BMC PlantBiol. 12:43

Walker RD, Doerfler J. 2009. Wild rice: the Minnesota legislature, a distinctive crop, GMOs, and Ojibweperspectives. Hamline Law Rev. 32:499–527

Wang F, Yuan QH, Shi L, Qian Q, Liu WG, et al. 2006. A large-scale field study of transgene flow fromcultivated rice (Oryza sativa) to common wild rice (O. rufipogon) and barnyard grass (Echinochloa crusgalli ).Plant Biotechnol. J. 4:667–76

Warwick SI, Legere A, Simard MJ, James T. 2008. Do escaped transgenes persist in nature? The case of anherbicide resistance transgene in a weedy Brassica rapa population. Mol. Ecol. 17:1387–95



Ann

u. R

ev. E

col.

Evo

l. Sy

st. 2

013.

44. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsite

it va

n A

mst

erda

m o

n 10

/10/

13. F

or p

erso

nal u

se o

nly.


Warwick SI, Stewart CN Jr. 2005. Crops come from wild plants—how domestication, transgenes, and linkagetogether shape ferality. In Crop Ferality and Volunteerism, ed. J Gressel, pp. 9–30. Boca Raton, FL: CRCPress

Weeden NF. 2007. Genetic changes accompanying the domestication of Pisum sativum: Is there a commongenetic basis to the ‘Domestication Syndrome’ for legumes? Ann. Bot. 100:1017–25

Wegier A, Pineyro-Nelson A, Alarcon J, Galvez-Mariscal A, Alvarez-Buylla ER, Pinero D. 2011. Recentlong-distance transgene flow into wild populations conforms to historical patterns of gene flow in cotton(Gossypium hirsutum) at its centre of origin. Mol. Ecol. 20:4182–94

Whitkus R, Doebley J, Wendel JF. 1994. Nuclear DNA markers in systematics and evolution. In DNA-BasedMarkers in Plants, ed. RL Philips, IK Vasil, pp. 116–41. Dordrecht: Kluwer

Whitlock MC, McCauley DE. 1999. Indirect measures of gene flow and migration: FST �= 1/(4N m + 1).Heredity 82:117–25

Whitney KD, Randell RA, Rieseberg LH. 2006. Adaptive introgression of herbivore resistance traits in theweedy sunflower Helianthus annuus. Am. Nat. 167:794–807

Whitney KD, Randell RA, Rieseberg LH. 2010. Adaptive introgression of abiotic tolerance traits in thesunflower Helianthus annuus. New Phytol. 187:230–39

Whitton J, Wolf DE, Arias DM, Snow AA, Rieseberg LH. 1997. The persistence of cultivar alleles in wildpopulations of sunflowers five generations after hybridization. Theor. Appl. Genet. 95:33–40

Wijnheimer EHM, Brandenburg WA, Ter Borg SJ. 1989. Interactions between wild and cultivated carrots(Daucus carota L.) in the Netherlands. Euphytica 40:147–54

Wilkinson MJ, Elliott LJ, Allainguillaume J, Shaw MW, Norris C, et al. 2003. Hybridization between Brassicanapus and B. rapa on a national scale in the United Kingdom. Science 302:457–59

Wilson GA, Rannala B. 2003. Bayesian inference of recent migration rates using multilocus genotypes. Genetics163:1177–91

Wright S. 1931. Evolution in Mendelian populations. Genetics 16:97–159Xia H-B, Wang W, Xia H, Zhao W, Lu B-R. 2011. Conspecific crop-weed introgression influences evolution

of weedy rice (Oryza sativa f. spontanea) across a geographical range. PLoS ONE 6:e16189Yang X, Xia H, Wang W, Wang F, Su J, et al. 2011. Transgenes for insect resistance reduce herbivory and

enhance fecundity in advanced generation of crop-weed hybrids of rice. Evol. Appl. 4:672–84



Ann

u. R

ev. E

col.

Evo

l. Sy

st. 2

013.

44. D

ownl

oade

d fr

om w

ww

.ann

ualr

evie

ws.

org

by U

nive

rsite

it va

n A

mst

erda

m o

n 10

/10/

13. F

or p

erso

nal u

se o

nly.

e i c n introgression of crop alleles into wild or weedy

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